1. Field of the Invention
This invention relates generally to electromagnetic transducers of the that may be employed as electro-acoustical drivers for loudspeakers. More particularly, the invention relates to electromagnetic transducers and loudspeakers having at least two coils capable of being driven by at least a two channel amplifier.
2. Related Art
An electro-acoustical transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. The basic designs and components of various types of electro-acoustical transducers are well-known and therefore need not be described in detail. An electro-acoustical transducer typically includes mechanical, electromechanical, and magnetic elements to effect the conversion of an electrical input into an acoustical output. For example, the transducer typically includes a magnetic assembly, a voice coil, and a diaphragm. The magnetic assembly and voice coil cooperatively function as an electromagnetic transducer (also referred to as a driver or motor). The magnetic assembly typically includes a magnet (typically a permanent magnet) and associated ferromagnetic components—such as pole pieces, plates, rings, and the like—arranged with cylindrical or annular symmetry about a central axis. By this configuration, the magnetic assembly establishes a magnetic circuit in which most of the magnetic flux is directed into an annular (circular or ring-shaped) air gap (or “magnetic gap”), with the lines of magnetic flux having a significant radial component relative to the axis of symmetry. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around a coil former. The coil former and the attached voice coil are inserted into the air gap of the magnetic assembly such that the voice coil is exposed to the static (fixed-polarity) magnetic field established by the magnetic assembly. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves. The diaphragm includes a flexible or compliant material that is responsive to a vibrational input. The diaphragm is suspended by one or more supporting elements of the loudspeaker (e.g., a surround, spider, or the like) such that the flexible portion of the diaphragm is permitted to move. The diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the coil former on which the voice coil is supported.
In operation, electrical signals are transmitted as an alternating current (AC) through the voice coil in a direction substantially perpendicular to the direction of the lines of magnetic flux produced by the magnet. The alternating current produces a dynamic magnetic field, the polarity of which flips in accordance with the alternating waveform of the signals fed through the voice coil. Due to the Lorenz force acting on the coil material positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves trough a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level, and are a function of the characteristics of the electrical signals supplied to the voice coil.
The energy transmitted by a speaker to sound waves is a function of the amount of movement of the diaphragm. The movement of the diaphragm is a function of the frequency of sound being transmitted (how frequently the diaphragm changes directions of movement) and the electrical voltage applied to the coil. The range of movement of the diaphragm is a function of the axial movement of the voice coil. This axial movement is often called the excursion.
For a loudspeaker to provide high output or deep bass, the loudspeaker may need a substantial excursion of the voice coil. In this context, an excursion is an axial movement of the voice coil from the position it assumes without electrical stimulus. Voice coils undergo excursions both towards and away from the diaphragm as the alternating electric current in the voice coil interacts with the magnetic field.
Due to advantages such as lighter weight and higher power handling, dual-coil/dual magnetic gap designs have been supplanting single-coil designs in loudspeakers. Many dual-coil/dual-gap designs are able to produce more power output per transducer mass and dissipate more heat than conventional single-coil designs. In a dual-coil driver, the voice coil includes two separate windings axially spaced from each other to form two coils, although the same wire may be employed to form both coils. In general, the magnet assembly of a dual-coil driver includes a stacked arrangement in which a magnet is axially interposed between a front pole piece and a rear pole piece. An outer ring is annularly disposed about the stacked arrangement such that all annular magnetic gap is defined between the outer ring and the stacked arrangement. The two coils are wound around a coil former and inserted into the gap such that one coil is located between the front pole piece and the outer ring and the other coil is located between the rear pole piece and the outer ring, in effect providing two magnetic gaps axially spaced from each other. As both coils provide forces for driving the diaphragm, the power output of the loudspeaker may be increased without significantly increasing size and mass.
The dual-coil configuration provides more coil surface area as compared with many single-coil configurations, and thus ostensibly is capable of dissipating a greater amount of heat at a greater rate of heat transfer. For example, a dual-coil design that doubles the surface area and number of turns of the coil winding may increase (e.g., nearly double) the capacity of the coil to dissipate heat. However, insofar as a desired advantage of the dual-coil driver is its ability to operate at a greater power output, so operating the dual-coil driver at the higher power output concomitantly causes the dual-coil driver to generate more heat. Hence, the improved heat dissipation inherent in the dual-coil design may be offset by the greater generation of heat.
In typical dual-coil dual gap driver, both voice coils are wired either in series or in parallel and attached to one amplifier channel. To achieve maximum power, it is also common to bridge the one amplifier channel with a second amplifier to supply the greatest voltage swing to a driver. In a powered speaker, the amplifier is built into the loudspeaker and the determination of whether to wire the voice coils in series or in parallel is predetermined. In other design, where an external amplifier must be utilized to drive the loudspeaker, separate leads may be wired to each voice coil to allow the user to make an independent determination whether to wire the voice coils to the amplifier in series or in parallel.
While numerous designs exist for dual-coil, dual gap drivers, a continuing need exist to design high-power, cost effective dual-coil dual gap transducers. A need further exists for a dual-coil, dual magnetic gap transducer design that not only allows for large excursions without extreme distortion, but that also reduces some of the common problems that occur with a loudspeaker, including, but not limited to, the generation of resistive heat within a loudspeaker.